FDP tendons !

The flexor digitorum profundus tendons run deepest to the fingertips; they are the only tendons that bend the end joints of the fingers.

Official name: Flexor digitorum profundus tendons

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What it is

Four deep tendons that pass through the carpal tunnel and through the FDS split to the fingertips.

What it does

Bend the fingertips; the only muscle that bends the end joints.

Key facts

Attaches: Four tendons through the carpal tunnel deep to the FDS, through the FDS split to the base of the distal phalanx of fingers 2–5; the lumbricals arise from them
Function: The only flexors of the distal interphalangeal joints; avulsion from the distal phalanx is jersey finger, most often the ring finger
Tight in: Finger and wrist extension

Relations

Course: Four tendons pass through the carpal tunnel behind the FDS, give origin to the lumbricals in the palm, enter the fibrous sheaths, come through the FDS split and run under the A4 and A5 pulleys to the distal phalanx
Superficial: FDS tendons, until it emerges through their split; beyond that it is the only flexor tendon in the finger
Deep: Floor of the carpal tunnel (carpal bones and palmar wrist ligaments); in the palm the deep palmar arch, the deep branch of the ulnar nerve and the interossei

How to find it

With the middle phalanx held so the PIP cannot move, bending the fingertip confirms the FDP of that finger. In a jersey finger a tender lump may be felt along the finger or in the palm where the stump has retracted.

Examination

Isolated FDP test: the examiner holds the PIP straight and asks for fingertip flexion; no DIP flexion means the FDP is cut or avulsed
Resting cascade: the fingers normally rest more bent from index to little; one finger lying straighter than the rest suggests a divided flexor tendon
'OK' sign: a flat pinch with a straight index DIP and thumb IP points to an anterior interosseous nerve palsy rather than a single tendon injury

On imaging

X-rays look for a bone fragment from the base of the distal phalanx in jersey finger and show how far it has moved; ultrasound or MRI locates the stump when no bone is attached.

Common problems

Jersey finger: the FDP tendon is pulled off the fingertip when grabbing a jersey; the fingertip cannot bend.
Cuts on the palm side of the finger or palm can divide flexor tendons.
Flexor tendon sheath infection is an emergency.

How it is diagnosed

Testing each finger joint's bending separately. X-rays look for a bone fragment; ultrasound if unclear.

Treatment

Surgical repair, ideally within days, followed by months of hand therapy.
Tendon graft or staged reconstruction for late or complex injuries.

Clinical notes

Flexor sheath infection is recognised by Kanavel's signs: finger held slightly bent, uniform swelling, tenderness along the sheath and pain on passive extension
Quadriga: a repaired or amputation-shortened FDP tendon limits flexion of the neighbouring fingers
Lumbrical-plus finger: after division of FDP beyond the lumbrical origin, trying to flex pulls through the lumbrical and paradoxically extends the IP joints
Zone I injuries lie beyond the FDS insertion and involve the FDP alone

Anatomical variants

The ring and little finger tendons often share a belly; the index tendon usually has an independent one

Test yourself

Think of the answer, then tap the question to see it.

What joint can only the FDP tendons flex?

The distal interphalangeal joints of fingers 2 to 5.

What is jersey finger, and which finger is most often affected?

Avulsion of the FDP tendon from the distal phalanx; most often the ring finger.

What is the quadriga effect of the FDP tendons?

The middle, ring and little FDP share a belly, so holding back or shortening one limits flexion of the others.

What are Kanavel's signs of flexor sheath infection?

Finger held slightly bent, uniform swelling, tenderness along the sheath and pain on passive extension.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): tendines musculi flexoris profundi digitorum TA2 2491
Official Spanish (DeCS): Tendones del músculo flexor profundo de los dedos
English (TA2): Flexor digitorum profundus tendons

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Flexor Digitorum Profundus Muscle; Anatomy, Shoulder and Upper Limb, Hand Long Flexor Tendons and Sheaths.

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Representative model, for educational purposes only.

FDP tendons highlighted on the Durgical 3D model